1. Field of the Invention
The present invention relates to a process and apparatus for generating high combustion temperatures and providing increased process efficiency utilizing internal radiant energy recuperation. Combustion air is introduced into a combustion chamber through an inlet porous plate, and gaseous exhaust products are withdrawn through an opposing outlet porous plate to provide radiant energy recovery and radiant energy transfer across the combustion chamber. The high temperature combustion process and apparatus of the present invention may achieve super-adiabatic combustion temperatures and are especially suitable for use in applications such as incineration.
2. Description of the Prior Art
In general, heat transmission by radiation and utilization of infrared energy has many advantages over conventional means of heat transmission by convection and conduction, particularly for many industrial applications. The operation and construction of infrared burners and radiant heaters is relatively simple, and thus more economical than other types of heat generation means. The intensity of radiant heat may be precisely controlled to provide greater efficiency, and infrared energy may be-focused, reflected, or polarized, in accordance with the laws of optics. Conventional gas fired infrared burners utilize flame energy or hot gases to heat a radiating refractory or other material, and thereby produce an approximately flat flame on or above the radiating surface.
Several types of gas fired infrared generators are currently available. Radiant tube burners comprise internally fired radiation units wherein the radiating surface is interposed between the flame and the load. Surface combustion infrared burners have a radiating burner surface comprising a porous refractory. The combustion mixture is conveyed through the porous refractory and burns above the surface to heat the surface by conduction. A third type of gas fired infrared generator comprises a burner having a radiating refractory surface heated directly with a gas flame. A fourth type of infrared generator utilizes a porous catalyst bed to oxidize fuel at low temperatures in a low temperature catalytic burner.
U.S. Pat. No. 3,810,732 teaches non-catalyzed flameless combustion wherein a perforated, sintered block forms one wall of a mixing chamber. The '732 patent teaches that temperatures on the order of about 1000.degree. C. to 2000.degree. C. are attainable by passing combustible mixture through the perforated, sintered block at a low flow rate until a temperature of about 950.degree. C. is reached, and then increasing the flow rate about ten to fifty times to cause combustion in the sintered block. The '732 patent also teaches a burner arrangement wherein two or three perforated sintered blocks are arranged parallel to one another and spaced apart, with combustion taking place in one of the blocks and radiating heat toward the adjacent block, thereby preheating the combustion air.
U.S. Pat. No. 3,217,701 teaches combustion at and within the surface of a porous fibrous refractory tube, with radiant energy and convection from exhaust gases directed to a secondary refractory surface to convert convection heat energy to radiant heat energy. U.S. Pat. No. 3,167,066 teaches radiant heating of fluids and gases by combustion in a perforated tubular section with solid refractory particles retained in an annular space surrounding the perforated tubular section and confined by a second, tubular radiating wall. U.S. Pat. No. 2,828,813 teaches a gas fired, porous wall furnace wherein passages are provided in the outer surface of the furnace wall to facilitate gas distribution and cooling. U.S. Pat. No. 4,087,962 teaches a direct heating surface combustor wherein spaced apart, flat plate burners form two walls of the combustion chamber with a radiant heat sink located between the plates. The '962 patent teaches low emission combustion and controlling of the combustion flame temperature to allow operation below adiabatic temperatures. U.S. Pat. No. 4,299,086 teaches combustion of a low heating value gas using less than a stoichiometric amount of oxygen in the presence of an oxygenation catalyst. U.S. Pat. No. 4,311,447 teaches a radiant surface combustor wherein a porous combustor element is in heat transfer relation with a heat transfer surface for absorbing radiant heat. U.S. Pat. No. 4,089,639 teaches premixing of water with fuel prior to burning to reduce-NO.sub.x formation.
Many different types and configurations of radiant gas burners are known to the art. U.S. Pat. No. 3,751,213 teaches a high intensity radiant gas burner having a ceramic honeycomb radiant element wherein combustion takes place within the cells of the honeycomb as well as in the combustion chamber. The material comprising the gas injection block, positioned downstream from the combustion chamber, is chosen on the basis of its density, taking into account the uniformity of gas flow, thermal insulating properties and durability of materials having various densities. Japanese Patent Publication No. 55025773 teaches an infrared burner having a honeycomb ceramic burner coated with an aqueous solution of magnesia-lithium silicate which is then fired to form a conductive layer. Combustion takes place at individual pores on the surface of the conductive layer, and the conductive layer promotes even heat distribution. U.S. Pat. No. 3,738,793 teaches an illumination burner having a layered porous structure, the layered pores maintaining a stable flame in a thoria-ceria illumination burner. Combustion does not occur within the pores of the combustor, but on the surface of the top layer. U.S. Pat. No. 3,912,443 teaches a layered ceramic radiant gas burner wherein the outer radiating layer comprises a coarsely porous ceramic material and an inner gas distributing layer comprises a finely porous, highly permeable ceramic material. U.S. Pat. No. 3,270,798 teaches a catalytic radiant burner having a lower density porous layer and a higher density porous layer, the lower density layer providing insulation and preventing flashback with flameless catalytic combustion occurring in the catalytic layers. U.S. Pat. No. 4,643,667 teaches a non-catalytic porous-phase combustor and process for generating radiant energy wherein the gas phase reaction and combustion take place within the pores of a multilayer porous plate having at least two discrete and contiguous layers, a first preheat layer comprising a material having a low inherent thermal conductivity and a second combustion layer comprising a material having a high inherent thermal conductivity and providing a radiating surface.
High temperature combustion, particularly for applications such as incineration, is an attractive industrial expedient to achieve better economics and greater waste destruction efficiencies. Combustion at high temperatures requires less time for equivalent waste conversion or, alternatively, permits greater waste conversion efficiencies in an equivalent combustor volume. Experimental approaches to achieve high temperature combustion for applications such as incineration have included techniques such as oxygen-rich firing of existing combustion apparatus, recuperative air preheat means, and thermochemical fuel modification. Each of these approaches requires more extensive equipment and/or more extensive investment in fuel required for combustion.